The present invention relates to hydraulic clutches of the wet type for use in power transmission systems to effect or interrupt power transmission, and more particularly to a double clutch serviceable as the main clutch of agricultural tractors and like vehicles equipped with drive wheels and power takeoff means.
Generally, single clutches of the dry type are widely used as the main clutch for interrupting or effecting transmission of the torque of engine to a speed change system. However, friction clutches of the dry type involve the inherent problem that because the play of the clutch pedal or the amount of idle travel of the clutch pedal tends to vary due to the abrasion of the disk, proper engagement and disengagement of the clutch requires frequent adjustment in accordance with variations in the amount of idle pedal travel. When the clutch is to be modified into a double clutch which requires a larger clutch case, it becomes difficult to accommodate the double clutch assembly in the single clutch housing, and there arises the necessity to employ another housing for the clutch assembly. Furthermore in systems in which a double clutch of the dry type is incorporated, means is included by which the clutch is automatically disengaged when a working implement such as rotary tiller has been lifted. With such system, however, the thrust bearing employed is subject to excessive thrust loads, which cause adhesion of the bearing owing to frictional heat, consequently breaking down the part in a short period of time. Thus the dry-disk double clutch involves problems in respect of durability.
Because of these problems, hydraulic friction clutches of the wet type which operate as immersed in oil are employed. Clutches of this type have the advantages of being low in coefficient of friction, permitting efficient dissipation of heat, involving reduced abrasion and being settable for the desired capacity when the relief pressure is changed, whereas hydraulic double clutches still have disadvantages, because according to conventional systems, the working oil taken in by a hydraulic pump is divided by a divider valve and fed to oil passages for the tractor-driving clutch unit and power takeoff clutch unit under the control of valves provided therefor respectively. Thus conventional systems require at least three valves which are expensive and render the overall construction costly. Moreover, the complex piping means necessary to supply the working oil to the clutch tends to cause pressure losses.
With hydraulic clutches of the wet type, the control valve provided at an intermediate portion of the oil passage for feeding the working oil to the clutch piston is opened or closed by a clutch pedal or like operating member by way of link means or the like. The valve conventionally used for this purpose functions in direct fashion merely to open or close the oil passage, only permitting interruption of the torque from the drive member or transmission of the torque therefrom to the driven member. It is therefore very difficult to bring the clutch into an intermediate state between engagement and disengagement, namely into semi-engaged state.